Auroras appear on Earth as ghostly displays of colorful light in the night sky, usually near the poles. Our rocky neighbor Mars has auroras too, and NASA’s MAVEN spacecraft just found a new type of Martian aurora that occurs over much of the day side of the Red Planet, where auroras are very hard to see. (…)
The MAVEN team was studying Mars’ atmosphere with the Imaging UltraViolet Spectrograph (IUVS), and observed that on occasion, the ultraviolet light coming from hydrogen gas in Mars’ upper atmosphere would mysteriously brighten for a few hours. They then noticed that the brightening events occurred when another MAVEN instrument, the Solar Wind Ion Analyzer (SWIA), measured enhanced solar wind protons.
But two puzzles make this type of aurora seem impossible at first glance: how did these protons get past the planet’s “bow shock,” a magnetic obstacle which normally diverts the solar wind’s charged particles around the planet? And how could the protons give off light, since atoms need electrons to do so?
“The answer was thievery,” said Justin Deighan, of the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder, lead author of a paper on this research appearing July 23 in Nature Astronomy. “As they approach Mars, the protons coming in with the solar wind transform into neutral atoms by stealing electrons from the outer edge of the huge cloud of hydrogen surrounding the planet. The bow shock can only divert charged particles, so these neutral atoms continue right on through.” [More at links]